Optical communication system

ABSTRACT

In networks carrying existing optical traffic on one wavelength band in combination with wavelength division multiplexed traffic carried on a second wavelength band, there is a need to enable processing of the two systems without subjecting the WDM channels to unacceptable losses. The invention meets the above need by the provision of a node in an optical communications network that has a first set of add/drop filter elements for extracting and combining optical signals carried on wavelength division multiplexed channels in a first wavelength band and an extraction element and combining element for dropping and adding, respectively, a service channel associated with the wavelength division multiplexed channels. The extraction element is arranged upstream of the add/drop filter elements relative to the direction of traffic flow and the combining element is arranged downstream of the add/drop filter elements. The extraction and combining elements are additionally adapted to drop and add, respectively, at least one further wavelength band carrying at least one optical traffic data channel.

CROSS REFERENCE TO RELATED APPLICATION

This application for patent claims the benefit of priority from U.S.Provisional Patent Application No. 60/232,271, which was filed on Sep.13, 2000.

FIELD OF INVENTION

The invention relates to optical communication systems. It hasparticular relevance to communication systems carrying WDM trafficchannels.

BACKGROUND ART

Optical communication systems employing wavelength divisionmulitiplexing (WDM) use a single fibre to carry multiple trafficchannels within a predetermined wavelength band. WDM systems are beingdeployed increasingly to optimise the transmission capacity of existingnetworks. Conventional WDM systems typically operate in a wavelengthwindow centred around 1550 nm. They also typically use at least oneservice channel for carrying system information, for example networkcontrol information and control signals. This channel is often carriedon the same optical fibre as the traffic information, but outside thetraffic carrying bandwidth. For example, a service channel commonly usedwith the 1550 waveband is carried on the 1510 nm wavelength.

Many optical systems in use today operate in the 1300 nm window. Anexample is the SONET/SDH. While WDM systems have obvious advantages,their introduction involves considerable investment as new equipmentmust be installed and maintained even in existing networks. The processis therefore a gradual one. When introducing WDM traffic to existingnetworks, operators are often obliged to maintain the previous service,at least for a time. The simultaneous operation of a WDM systemoperating in the 1550 nm window with other services centred round the1300 nm waveband on the same network, and within the same nodes,requires certain measures to prevent interference between the twoservices. This includes the installation of filter components, which maycomprise simple fused couplers or thin film filters for separating the1300 nm waveband from the 1550 nm waveband. In non-amplified WDMsystems, filter components are necessary for separating out the servicechannel at each node. However, each filter component causes a basicpower loss to channels passively relayed through the filter. Theadditional 1550 nm/1300 nm couplers along any single link will naturallyincrease the total link loss, in some cases to a level that exceeds theallowable link loss, such that the WDM system cannot be utilised overthe whole network.

There is consequently a need for an arrangement that can allow WDMsystems operating in a first wavelength band to be used on existingoptical networks at the same time as optical systems operating in asecond, different wavelength band.

SUMMARY OF INVENTION

The invention meets the above need by the provision of a node in anoptical communications network that has a first set of add/drop filterelements for extracting and combining optical signals carried onwavelength division multiplexed channels in a first wavelength band andan extraction element and combining element for dropping and adding,respectively, a service channel associated with the wavelength divisionmultiplexed channels. The extraction element is arranged upstream of theadd/drop filter elements relative to the direction of traffic flow, andthe combining element is arranged downstream of the add/drop filterelements. The extraction and combining elements are additionally adaptedto drop and add, respectively, at least one further wavelength bandcarrying at least one optical traffic data channel.

By combining the adding and dropping of traffic channels that areseparate from the WDM wavelengths with that of the service channel, theWDM traffic can be extracted, processed and combined with thetransmission path in the normal manner without the need for additionalfilter elements to prevent interference and without the additionalimposed losses associated therewith.

The traffic channels carried on the second wavelength band arepreferably not wavelength division multiplexed channels. However, thismay not be the case, and the separation of the two bands prior to thedropping and adding of individual channels means that a secondwavelength band carrying WDM traffic will likewise be protected from theadditional loss of the WDM add/drop filter elements.

The extraction element is connected to a splitting arrangement forseparating the service channel wavelength from the second wavelengthband. The separated wavelengths can then be converted to electricalsignals and processed separately. The second wavelength band and servicechannel wavelength are furthermore combined using a coupling arrangementwhich then relays the combined signals to the combining element.

Advantageously, a bypass path can be provided for the second wavelengthband between the splitting arrangement and the coupling arrangement. Thesecond wavelength band is thus merely removed from the transmission pathto allow the WDM channels to be extracted from the transmission path,processed and added to the transmission path.

The second wavelength band is preferably arranged on the same side ofthe wavelength spectrum as the service channel wavelength relative tothe first wavelength band, as this greatly simplifies the constructionof the extraction and combining elements.

Preferably, the first wavelength band is centred around 1550 nm, whilethe second wavelength band is centred around 1300 nm. The servicechannel is preferably carried at 1510 nm.

The invention also resides in an optical network including nodes asdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the present invention will becomeapparent from the following description of the preferred embodimentsthat are given by way of example with reference to the accompanyingdrawings. In the figures:

FIG. 1 schematically illustrates an optical network,

FIG. 2 schematically illustrates part of a node in an opticalcommunications network according to a first embodiment of the presentinvention, and

FIG. 3 schematically illustrates part of a node according to a secondembodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an optical communications network comprising a number ofnodes 10 connected by a bidirectional transmission path 20 constitutedby optical fibres. In the simplified network shown in FIG. 1, the nodes10 are connected to form a ring configuration, however it will beunderstood that the present invention may be employed in other networkarchitectures.

Two communications systems are carried by the illustrated network. Thefirst is a system carrying user traffic in the wavelength band centeredaround 1300 nm, for example the SONET/SDH. The other system is awavelength division multiplexed (WDM) system operating in the 1550 nmwindow. An optical service channel OSC carried at 1510 nm providessystem information, such as network monitoring and control informationto and from each node 10 for the WDM system. Each node 10 comprisesadd/drop filter components for combining or extracting channels ofspecified wavelengths to and from the transmission path. Each wavelengththat is not dropped by a filter component is passively relayed by thecomponent, either by transmission or reflection, with a finite loss. Theoptical service channel OSC is also dropped at each node 10 that servesas a WDM node, i.e. a node that drops or adds WDM traffic.

Turning to FIG. 2, there is shown a schematic representation of part ofa node 10 according to a first embodiment of the present invention. Thefigure specifically shows the arrangement of add/drop elements for anode wherein traffic channels carried in the 1300 nm wavelength windowdenoted by B₁₃₀₀ and wavelength division multiplexed traffic channelscarried in the 1550 nm window denoted by B_(WDM) are added to thenetwork and dropped from the network. Since the node 10 serves WDMtraffic, the optical service channel carried at wavelength 1510 nm anddenoted by λ_(OSC) is likewise dropped and added at the node 10.

The data carried by wavelengths B₁₃₀₀, B_(WDM) and λ_(OSC) arrives atthe node 10 via the optical fibre transmission path 20. Only a singleunidirectional fibre is shown in FIG. 2. It will be understood, however,that a second fibre may be provided for carrying information in thereverse direction. A first drop filter 100 is connected to the inputfibre 20 for extracting the 1330 nm wavelength window and the OSCchannel. In the present embodiment, the OSC channel is carried at 1510nm. The OSC channel and the 1300 nm traffic are thus carried on the sameside of the wavelength spectrum relative to the WDM traffic and can bedropped from the transmission path 20 using filter having a simple bandpass or high-pass frequency transmittance characteristic. For a simplehigh-pass frequency characteristic, the cut-off frequency would bechosen to correspond to a suitable wavelength below, i.e. shorter than,the wavelength of the 1550 nm window. Naturally, if the 1300 nm ornon-WDM traffic and the OSC channel were carried on wavelengths locatedon opposite sides of the 1550 nm wavelength window, the drop filter 100would be designed to have a high transmittance at the non-WDM trafficwavelengths and the OSC wavelength and a low transmittance in the 1550nm window. The filter 100 is preferably an interference filter, such asa thin film filter, but other filter designs are also possible, forinstance a fibre grating filter such as a Bragg grating filter.

After extraction from the transmission path 20, the non-WDM channelscarried on the wavelength band B₁₃₀₀ and the OSC channel at λ_(OSC) arefed to a further filter 110 which separates the wavelength band B₁₃₀₀from the OSC channel wavelength λ_(OSC). Filter 110 can be the same typeas filter 100, i.e. preferably an interference filter. The non-WDMtraffic is then processed in the usual manner by non-shown circuitry inthe node 10. Information on the OSC channel is likewise received andprocessed by specific, non-shown circuitry as is generally known in theart.

The WDM channel wavelengths are passively relayed at filter 100 andundergo a finite power loss thereby. Depending on the filter technologyutilised, the WDM channel wavelengths will be either reflected ortransmitted by the filter 100. Additional add/drop filters 120, 130 arearranged on the transmission path for the WDM wavelengths, which for thesake of example will be denoted as λ₁, λ₁, . . . λ_(n). These may takethe form of multiple add/drop filters, where each filter is designed todrop and/or add a single channel wavelength λ₁, λ₂, . . . λ_(n), fromamong all the WDM channels. Alternatively, the filters 120, 130 may takethe form of a demultiplexer and multiplexer, respectively, fordropping/adding multiple channels λ₁, λ₂, . . . λ_(n) together andsubsequently separating these into the individual wavelengths. It willbe appreciated that any combination of wavelengths may be dropped andadded by the elements 120 and 130 depending on the connections requiredin the network. The possible configurations are generally known in theart and will not be described further here.

Downstream of the final add filter 130 non-WDM traffic and the OSCchannel are again added to the transmission path 20 using a coupler 140.The wavelengths of the non-WDM traffic B₁₃₀₀ and the OSC channel λ_(OSC)are combined upstream of this coupler 140 in a separate coupler 150. Thecouplers 140, 150 are preferably of the same type and design as filters100 and 10, respectively, but may be of any suitable structure.

By dropping and adding both the non-WDM traffic and the OSC channel withthe same add/drop filter the loss suffered by the WDM traffic channelsis limited to that imposed by a single filter. This is of greatadvantage for non-amplified WDM systems, since each additional add/dropfilter on the transmission path imposes a finite power loss on thechannels. Each loss in power is equivalent to a specific length ofoptical fibre, so ultimately the number of add/drop elements on atransmission path limit the useful size of a network for WDM traffic. Inconventional WDM systems the OSC channel is generally dropped forprocessing before the WDM traffic channels and recombined with thetransmission path after any WDM channels have been added. Combining theextraction and addition of the non-WDM traffic channels with that of theOSC channel allows the network to be utilised simultaneously fordifferent optical communication systems without imposing additionalpower loss on WDM channels.

In a further embodiment of the present invention the non-WDM traffic isnot processed at the node 10 but merely transmitted through the node.This is shown schematically in FIG. 3. The arrangement in FIG. 3 isalmost identical to that of FIG. 2 with the exception that the non-WDMtraffic at the 1300 wavelength band B₁₃₀₀ is not extracted forprocessing, but is instead transmitted directly to the coupler 150 forcombining with the new information on the OSC channel λ_(OSC). Thenon-WDM traffic is thus essentially bypassed. In addition to limitingthe power loss imposed on the WDM channels, this arrangement alsopermits the standard WDM add/drop elements 120, 130 to be used. If thenon-WDM traffic channels in the 1300 nm wavelength band were transmittedthrough the node with the WDM channels, the filters 120, 130 would needto be specially adapted to prevent the two wavelength bands B₁₃₀₀ andB_(WDM) from interfering with one another.

1. A node in an optical communication network, said node being connectedin a transmission path for carrying multiple traffic data channelsincluding wavelength division multiplexed channels carried in a firstwavelength band and at least one service channel associated with saidwavelength division multiplexed channels and carried on at least onefurther wavelength separate from said first wavelength band, said nodeincluding a set of first filter elements for adding at least one of saidwavelength division multiplexed data channel wavelengths to saidtransmission path and dropping at least one of said wavelength divisionmultiplexed channel wavelengths from said transmission path, anextraction element for dropping said at least one service channelwavelength from said transmission path, said extraction element beingarranged upstream of said first set of filter elements, a splittingmeans arranged to receive optical signals from said extraction elementand to separate said service channel wavelength from at least one secondwavelength band, wherein said splitting means are connected to saidcoupling means via a bypass path for relaying signals carried on said atleast one second wavelength band from said splitting means to saidcoupling means, and a combining element for adding said at least oneservice channel wavelength to said transmission path, said combiningelement being arranged downstream of said set of first set of filterelements, wherein said extraction and combining elements are adapted todrop and add, respectively, said at least one second wavelength band inaddition to said at least one service channel wavelength, and passivelyrelay said first wavelength band, said at least one second wavelengthband being separate from said first wavelength band and carrying atleast one optical traffic data channel.
 2. A node as claimed in claim 1,wherein said at least one second wavelength band carriesnon-wavelength-division-multiplexed traffic channels.
 3. A node asclaimed in claim 1, wherein said at least one service channel wavelengthand said at least one second wavelength band are arranged on the sameside of the wavelength spectrum relative to said first wavelength band,wherein said extraction element and said combining element drop and add,respectively all wavelengths on the side of the spectrum containing saidservice channel wavelength and second wavelength band.
 4. A node asclaimed in claim 1, further comprising said coupling means arranged tofeed optical signals to said combining element and to couple saidservice channel wavelength with said second wavelength band.
 5. A nodeas claimed in claim 1, wherein said first wavelength band is centeredaround 1550 nm and said second wavelength band is centered around 1300nm.
 6. A node as claimed in claim 5, wherein said service channel iscarried at 1510 nm.
 7. An optical communications network for carrying afirst wavelength band carrying wavelength division multiplexed opticaldata channels and a second wavelength band carrying at least one opticalservice channel associated with said wavelength division multiplexedchannels, comprising: optical nodes connected to a transmission path,each optical node having a first set of add/drop elements for adding anddropping optical data channels carried in said first wavelength band andadditional add and drop elements for adding and dropping, respectively,said at least one optical service channel carried in said secondwavelength band, wherein said additional drop element is arrangedupstream of said first set of add/drop elements and said additional addelement is arranged downstream of said first set of add/drop elements,wherein said communication network carries a third wavelength bandcarrying optical traffic data, wherein said additional add and dropelements are arranged to add and drop at least said third wavelengthband in addition to said second wavelength band, wherein each saidoptical node includes a bypass path for said third wavelength bandconnecting said splitting means to said second add element via acoupling means; said splitting means arranged to receive optical signalsfrom said additional drop element and to separate said second wavelengthband from said third wavelength band.
 8. A network as claimed in claim7, further comprising said coupling means arranged to feed opticalsignals to said additional add element and to couple signals carried onsaid second wavelength band with signals carried on said thirdwavelength band.